"John" wrote in message
...
I have come across this term in descriptions of wood. Does it refer to
changes with humidity, or something else?

A very much simplified explanation:

Wood is hydroscopic in nature and almost always contains water to some
extent, more as a growing plant, less as a dead plank. Wood fibers exchange
moisture with the surrounding air until an equilibrium is reached between
the air and the wood. These fibers, being oriented lengthwise for the most
part, expand across their width to a greater extent than they do length when
they absorb water. When moisture content is high, once again generally
caused by the relative humidity of the surroundings, and depending upon
species and how it was cut from the log, wood will swell across its grain,
sometimes as much as 1/4 ", or more, in a foot.

Conversely wood will shrink for the same reason when exposed to low relative
humidity for any length of time.

When building furniture, or anything made of wood or wood products, you must
take this usually seasonal change in humidity, and subsequent dimensional
instability, into account.

Much of the art and science of joinery deals with the propensity of wood to
be dimensionally unstable due to the relative humidity of its surroundings.

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Last update: 5/15/04